Electrodes for electrostatic fluid beds



Feb. 17, 1970 E. s. eooomoss ETAL 3,496,413

ELECTRODES FOR ELECTROSTATIC FLUID BEDS Filed March 24, 1967 FIG.7

ATTORNEYS United States Patent US. Cl. 3173 6 Claims ABSTRACT OF THE DISCLOSURE Electrodes for fluidized powder beds are provided. The electrodes are V-shaped electrodes extending in parallel relationship with the tips of the base legs forming a gap through which the powder passes. As the powder passes, the powder wipes the electrode surface to charge the powder.

This invention relates to an improved electrode arrangement for electrostatic fluidized beds.

Fluid bed apparatus is known to the art in which powder is held in suspension by air passing upwardly through a container. For coating of objects with such powder, the object must be inserted within the container.

In such apparatus, however, powder adherence to the object is not entirely satisfactory. For this reason, the art has developed such apparatus in which the individual powder particles are electrostatically charged. By maintaining the object to be coated at a different electrostatic level as for example, by grounding the object, the attractive force 011 the electrostatically charged particles will ensure a coating which is even and continuous over the entire object.

Electrostatically charging of the powder particles has, however, suffered from certain drawbacks in the arrangements known to the art, particularly with respect to the a electrode configuration for such charging. Large electrodes provide high capacitance. Thus, the charge on the electrode is very high and accidental discharges can have serious consequences. In addition, the electrode shape has not provided the desired powder charging capabilities particularly when fine powders are employed.

It is, therefore, an object of the present invention to provide an improved electrode for more elfective charging of powder contained in a fluid bed apparatus.

It is a further object of this invention to provide an improved electrode construction in which the deleterious effects of electrode capacitance is reduced.

In accordance with these objects, there is provided, in a preferred embodiment of the present invention, an electrode consisting of a plurality of V-shaped electrode elements extending transversely across the fluid bed for charging of the powders in said bed.

The V-shaped electrodes may be provided only with a conductive element at the adjacent edges of the V.

The electrodes may be mounted within the fluid bed or may be mounted on the top of hood enclosing the bed so that the powder is charged only as the powder escapes from the fluid bed container. This embodiment is more effective since the air suspension need not overcome the electrostatic charge.

The V-shaped electrodes may be coupled together and charged from a single high voltage source. Preferably, however, the electrodes are charged from a single source through blocking gates. In this case, the discharge capacitance on a short is limited to the capacitance of each V-shaped element of the electrode, reducing the total charge flowing through a short or accidental ground.

Having briefly described this invention, it will be described in greater detail along with other objects and advantages in the following portions of the specification, which may best be understood by reference to the accompanying drawings, of which:

FIG. 1 is a perspective view of one embodiment of the present invention;

FIG. 2 is a cross sectional view of another embodiment of the electrodes in accordance with the present invention;

FIG. 3 is a partially sectioned view of still another embodiment of the present invention;

FIG. 4 is a perspective view of still another embodiment of the present invention;

FIG. 5 is a schematic of the charging electrodes preferably employed in the present invention;

FIG. 6 is a plan view of another embodiment of a charging electrode element in accordance 'with the present invention during a stage of manufacture; and

FIG. 7 is a perspective view of the electrode of FIG. 6 after forming.

In FIG. 1, there is shown an electrode for a fluid bed apparatus, which electrode consists of a plurality of V- shaped segments 10 mounted on base members 12 and 14 by securing the apex 16 of each individual element to the respective base members 12 and 14. A typical electrode would be a 10 x electrode with the V-shaped segments securing the 15 direction. The V-shaped segments are positioned in parallel disposition with a gap 18 between adjacent edges of the base legs of the V-shaped elements 16. When installed in a fluid bed apparatus, the powder is blown upwardly through the electrode and as it is blown through the gap 18, the powder rubs the electrode segments and is charged thereby. The passage through the gap (approximately A across) also charges the powder. The combined effect provides very effective charging of even small particles.

In the example illustrated in FIG. 1, the V-shaped segments are constructed of a conductive element such as copper. To reduce the capacitance of the segments, the embodiment shown in FIG. 2 may be employed.

In FIG. 2, there is shown V-shaped electrode segments 20 formed of an insulator such as a plastic ma: terial. The adjacent edges of the base of each segment is provided with a conductive strip 22. In all other respects, the construction of FIG. 2 is the same as that of FIG. 1. The electrode capacitance of the construction of FIG. 2 is usually less than that of FIG. 1. However, it is found that, with the edge rendered conductive by applying a W strip of metal or by being metallized with a strip width of 7 powder charging is effective.

In FIG. 3, there is shown a typical installation in a fluid bed container having a porous plate 32 through which air flows from the air chamber 34 upwardly through the bed to suspend the powder particles in a fluid bed arrangement, as indicated by the line 36. As the powder particles pass the electrode segments 38, they are charged. In this arrangement, it is possible to mount the electrode assembly on guides 42 and to vibrate the entire electrode assembly by a vibrator 44 operating through shaft 46. The tank is sealed by a bellows 48 on the shaft 46 to prevent escape of powder. In this arrangement, vibration of the entire container may often be eliminated.

In FIG. 4, there is shown another embodiment in which the container 50 is enclosed with a baflle or hood 52. The electrode structure 54 is mounted on the top of the hood 52 and consists of V-shaped segments 56 which may be the type discussed in connection with FIGS. 1 and 2. The hood arrangement may be conductive or non-conductive, but it is preferable to charge only the electrode segments 56. By the hooded arrangement, all of the powder is forced through the gap 58 between electrode segments. Since the powder is not charged until passage through the gap, the suspending air current only is required to overcome the gravitational force on the individual particles. Thus, the arrangement shown in FIG. 4 is oftentimes startlingly more effective with small particle sizes such as particles of the order of 20 microns or less which are normally quite diflicult to charge and to maintain in a defined suspension.

The electrode segments as, for example, the segments of FIG. 1, may be charged as an entity as, for example by constructing the base members 12 and 14 of conductive material. However, it is preferred that the individual segments 10 be charged independently from thesource, as shown in FIG. 5.

In FIG. 5, there is shown the electrode segments 10 which are charged from a high voltage power supply 60 through impedances such as diodes 62, 64, 66. Respective bleeder resistors 68, 70, 72 are provided to leak off the charge when operations are terminated. By the arrangement shown in FIG. 5, however, the capacitance of each individual segment is charged. Thus, should the operator contact an electrode segment, the discharge will discharge only that electrode segment since the diodes will block discharge by the other segments. Thus, in this manner, the disadvantages of the high capacitance of an electrode having a relatively large surface area with concomitantly high charge stored thereon is avoided. Dropping impedances such as resistors may be used in place of the diodes to drop the discharge to low magnitudes.

Another embodiment having certain advantages in manufacture and use is shown in FIGS. 6 and 7.

In FIG. 6, there is shown an electrode element formed of a flat plate 76 of a thermoplastic strip such as a strip 15" long by 2 wide by thick. A thin conductive strip 78 is formed on one surface of the base material and follows a path including portions parallel to the edges of the base. The conductive strip may be formed by conventional techniques such as printed circuit etching or deposition of conductive paints. I prefer deposition processes since the path resistance can usually be maintained high by such processes.

The electrode element is then mechanically folded on the centerline to provide the V-shaped electrode element shown in FIG. 7. A score line may be cut into the back of the board along the fold line to eliminate stretching of the. conductive strips in applications where the board thickness causes overstressing of the conductive strips.

The electrode element shown in FIGS. 6 and 7 can be fabricated in simple and economical manner. In operation, the path resistance helps to overcome the problem of discharges from the electrode. If any part of the conductive strip is shorted as, for example, by operator contact, the path resistance comes into play in limiting the discharge current amplitude.

In most installations, it has been found that the current limiting capability of the element is such that other pro tective devices in the power supply or in the feed can be eliminated. Additional dropping can be obtained by feeding the paralleled elements in an installation through resistors to give similar protection to that offered by the gates of the embodiment shown in FIG. 5.

In FIGS. 6 and 7, a tortuous path has been illustrated. This path provides the desired total resistance with conventional processing. In some applications, it is found more desirable to run the conductive strip along the edge alone, similar to the embodiment of FIG. 2. However, to discharge protection, the path formation must be more accurately controlled to provide the desired high resistance.

What is claimed is:

1. A charging electrode for electrostatic charging of powder in a fluid bed apparatus comprising at least two V-shaped electrodes formed of nonconductive material, said electrodes being mounted with the apexes thereof positioned at spaced apart locations and with the base legs of adjacent electrodes defininga charging slot therebetween, each of said base legs having a strip of conductive material applied along the edge thereof defining said charging slot, and means to charge said electrodes to charge the powder sweeping upwardly along said electrodes and through said charging slot.

2. A charging electrode for electrostatic charging of powder in a fluid bed apparatus comprising at least two V-shaped electrodes, said electrodes being mounted with the apexes thereof positioned at spaced apart locations and with the base legs of adjacent electrodes defining a charging slot therebetween, means for vibrating said electrodes, and means to charge said electrodes to charge the powder sweeping upwardly along said electrodes and through said charging slot.

3. A charging electrode for electrostatic charging of powder in a fluid bed apparatus comprising at least two V-shaped electrodes, said electrodes being mounted with the apexes thereof positioned at spaced apart locations and with the base legs of adjacent electrodes defining a charging slot therebetween, and a source of high potential to charge said electrodes to charge the powder sweeping upwardly along said electrodes and through said charging slot, said source of high potential being connected to each of said electrodes through an independent respective impedance, said impedance resisting discharge from its respective electrode through another electrode to minimize the total potential discharge from any electrode.

4. A charging electrode for electrostatic charging of powder in a fluid bed apparatus comprising at least two V-shaped electrodes, a hood over the bed, and means to charge said electrodes to charge powder sweeping upwardly therealong, said electrodes being mounted in said hood with the apexes thereof positioned at spaced apart locations and with the base legs of adjacent electrodes defining therebetween, at the top of said hood, a charging slot through which said powder passes.

5. A charging electrode for electrostatic charging of powder in a fluid bed apparatus comprising at least two V-shaped electrodes, said electrodes being formed of a nonconductive base material having a strip of high resistance conductive material applied on one side thereof in a path having portions extending parallel to and close to the edges thereof, said electrodes being mounted with the apexes thereof positioned at spaced apart locations and with the base legs of adjacent electrodes defining a charging slot therebetween, and means to charge said electrodes to charge the powder sweeping upwardly along said electrodes and through said charging slot.

6. Electrostatic fluid bed apparatus comprising a source of powder, means for suspending the powder in a fluid bed arrangement, a charging electrode, and means for charging said electrode, said charging electrode comprising at least two V-shaped electrodes motmted with the apexes thereof directed toward the bottom of said fluid bed and at spaced apart locations, the base legs of adjacent electrodes defining a charging slot therebetween to charge powder sweeping upwardly along said electrodes and thence through said charging slot.

References Cited UNITED STATES PATENTS 2,869,511 1/1959 Dickey et al. 118-638 2,976,839 3/1961 Okma et a1 118640 XR 3,408,986 11/1968 Walsh et al 3l7-3 XR LEE T. HIX, Primary Examiner W. M. SHOOP, JR., Assistant Examiner US. 01. X.R. 

